Hi Michael,
There are multiple scenarios:
A) If the E590 has an SVP (Service Processor):
1) The username and password you are using does not exist on the SVP.
2) If the E590 code and SVP version are old then there may be a problem where the SVP needs to be rebooted to recover communicaitons.
B) If the E590 is SVP-less:
1) The E590 LAN may need to be reset, open a hardware case and ask for your local Customer Engineer to be dispatched to reset the LAN card.
2) The GUM Controllers may need to be rebooted. Again, open a hardware case as in (1) above.
C) firewalls or other network issue:
1) Launch Putty and try to SSH to the IP of the GUM controllers using port 20522. If it never asks for username there is a firewall or other network issue.
Hope that helps.